In the past, when using an electrodischarge machine for forming a fine hole in a workpiece, there has been known an apparatus designed to judge a need for replacement of an electrode while considering the ratio of consumption of an electrode for electrodischarge machining and a machining depth (plate thickness of workpiece) (for example, see PLT 1). In the apparatus which is described in this PLT 1, the required electrode length which is required for machining is calculated from a preset electrode consumption ratio and machining depth, an electrode length detecting means is used to detect a current electrode length, and, when the required electrode length is longer than the electrode length, it is judged that electrode replacement is necessary and advance to the electrodischarge machining process is stopped.
However, when using a turbine blade etc. as a workpiece, the plate thickness of the workpiece at the machining location is not necessarily constant. Therefore, if, like in the apparatus which is described in the above PLT 1, using a preset machining depth as the basis to calculate the required electrode length, the need for electrode replacement cannot be precisely judged. Further, there is waste in setting the electrode consumption ratio larger in view of safety and ending up replacing an electrode which can still be used.